The present invention relates to acoustic resonators, and more particularly, to resonators that may be used as filters for electronic circuits.
The need to reduce the cost and size of electronic equipment has led to a continuing need for ever-smaller electronic filter elements. Consumer electronics such as wireless telephones and miniature radios place severe limitations on both the size and the cost of the components contained therein. Further, many such devices utilize electronic filters that must be tuned to precise frequencies. Electronic filters allow those frequency components of electrical signals that lie within a desired frequency range to pass while eliminating or attenuating those frequency components that lie outside the desired frequency range. Such filters are referred to as bandpass filters.
One class of electronic filters that has the potential for meeting these needs is constructed from thin film bulk acoustic resonators (FBARS). These devices use bulk longitudinal acoustic waves in thin film piezoelectric (PZ) material. In one simple configuration, a layer of PZ material is sandwiched between two metal electrodes. The sandwich structure is preferably suspended in air. A sample configuration of an apparatus 10 having a resonator 20 (for example, an FBAR 20) is illustrated in FIGS. 1A and 1B. FIG. 1A illustrates a top view of the apparatus 10 while FIG. 1B illustrates a cut-away side view of the apparatus 10 along line A—A of FIG. 1A.
The resonator 20 is fabricated above a substrate 12. Deposited and etched on the substrate 12 are, in order, a bottom electrode layer 22, piezoelectric layer 24, and a top electrode layer 26. Portions (as indicated by brackets 20) of these layers that overlap and are fabricated over a cavity 14 constitute the resonator 20.
The electrodes 22 and 24 are conductors while the PZ layer 18 is typically piezoelectric material such as Aluminum Nitride (AlN).
When alternating current electric field is applied between the metal electrodes 22 and 26, the PZ layer 24 converts some of the electrical energy into mechanical energy in the form of mechanical waves. The mechanical waves propagate in the same direction as the electric field creating resonance at a particular resonant frequency. Ratio of the resulting mechanical energy to the electrical energy applied to the FBAR 20 is referred to as the coupling coefficient of the FBAR 20. Coupling coefficient of a resonator is determined, primarily, by the coupling coefficient of its PZ layer. Effective coupling coefficient is proportional to the intrinsic piezoelectric (a material constant) times a geometric term which is affected by thicknesses and locations of the different layers in the FBAR.
At the resonant frequency, the resonator 20 acts as an electronic resonator. The resonant frequency is determined by many factors including the total mass and thickness of the FBAR 20. Resonators for applications in the GHz range may be constructed with physical dimensions on the order of less than 100 microns in lateral extent 28 and a few microns in total thickness 29. In some implementation, for example, the resonator 20 is fabricated using known semiconductor fabrication processes and is combined with electronic components (not shown in the Figures) and other resonators (not shown in the Figures) to form electronic filters for electrical signals.
For a particular application of the FBAR 20, for example for 1 GHz to 2 GHz PCS bandpass filter applications for wireless communication devices, it is desirable to manufacture resonators having a particular desired coupling coefficient as well as having a particular desired resonant frequency.
Given the desired resonant frequency, one technique for achieving the desired coupling coefficient for a resonator is to select the PZ material, for its PZ layer, having coupling coefficient that is at or close to the desired coupling coefficient. For example, Aluminum Nitride (AlN), in a high quality crystalline form, has a coupling coefficient of approximately 6.5 percent. Accordingly, to manufacture a resonator having coupling coefficient of about 6.5 percent, the resonator can be fabricated with high quality AlN as its PZ layer.
However, this technique for achieving the desired coupling coefficient is not practical. This is because, at minimum, different PZ material needs to be discovered for each desired coupling coefficient value.
Another technique for achieving the desired coupling coefficient for a resonator is by varying the thickness of the PZ layer. For example, to realize a desired coupling coefficient of 3.7 percent for the FBAR 20, a thinner layer of high quality AlN can be deposited to form the PZ layer 24. Thinner PZ layer increases the resonant frequency of the FBAR 20. To maintain the desired resonant frequency, the electrode layers 22 and 26 need be made thicker to compensate for the loss of mass and thickness in the PZ layer 24.
Application of this technique for achieving the desired coupling coefficient results in resonators that are relatively more susceptible to frequency drift as temperature changes. This is because the electrodes 22 and 26 are made of material (such as, for example, Molybdenum) having a higher temperature coefficient than the PZ material (such as, for example, AlN). As the ratio of the mass of the electrodes to the mass of the PZ layer increases, the temperature coefficient of the resonator as a whole increases. Further, with a relatively thinner PZ layer, instances of undesirable electrostatic discharges (ESD) between the bottom electrode 22 and the top electrode 26 are increased compared to instances of such ESD for a resonator having a relatively thicker PZ layer.
Yet another technique for achieving the desired coupling coefficient is to reduce the quality of the PZ material. That is, to fabricate a resonator where the PZ material 24 has lower quality, or less order within its physical structure. For this reason, this technique can be called the “disordering technique.” With the decrease in the order within the molecular structure of the material of the PZ layer 24, the piezoelectric characteristic of the PZ layer is reduced thereby reducing the coupling coefficient of the PZ layer 24. For example, to realize a desired coupling coefficient of 3.7 percent, a lower quality AlN can be deposited.
However, in the manufacturing process, it is difficult to control the degree of disorder, or quality, of the PZ material such as AlN and to consistently reproduce the exact degree of disorder to realize the desired coupling coefficient. This is because there are many factors that need be tightly controlled to consistently reproduce the exact degree of disorder. These factors include, for example, base temperature, gas pressure, contamination of various portions of the process equipment, humidity, sputter rate, chemical mixture ratio, deposition temperature, substrate roughness, vacuum quality, sputter chamber geometry, crystalline structure, sputter power, and many other factors not all of which are controllable or even known.
Further, the degree of disorder, thus the resulting coupling coefficient, is sensitive to small variations in process or manufacturing factors. Slight variations in any one or more of these factors in the manufacturing process results in widely varying degrees of disorder.
Consequently, there remains a need for an improved method for controlling piezoelectric coupling coefficient in film bulk acoustic resonators.